Where a cavity to be machined in the workpiece is deep and narrow and/or of intricate shape, difficulty in enhancing the machining efficiency has always been the problem in the EDM art. Attempts have been made to resolve this problem by imparting ultrasonic vibrations to the machining electrode during the EDM machining operation. The prior technique has employed a single vibration horn assembly which has an electromechanical transducer energizable with a high-frequency oscillatory electrical signal to cause mechanical oscillations therein, and an amplifier horn member attached to the transducer to provide the mechanical oscillations of an amplified strength from its vibration output face which is reduced in cross-sectional area. Heretofore, such an ultrasonic horn assembly has commonly been attached vertically to the machine spindle or electrode support to locate the vibration output face of the horn downwards to which the machining electrode is in turn attached so as to depend therefrom, the machining electrode having the electrode machining surface positionable in spaced juxtaposition with the workpiece. The horn member or assembly is thus arranged as an integral part of the total assembly and coaxial with the spindle and the electrode. This arrangement has proved to be successful to increase the EDM machining efficiency to a measurable extent but has imposed restrictions on the material, shape and weight of the machining electrode to be carried by the horn assembly since the body of the electrode need to resonate with the vibrating horn carriage. If there is deviation from resonance or mechanical impedance is mismatched, the vibrational input to output efficiency is sharply reduced to such an extent as to render the assembly impractical. Furthermore, undesirable excessive heat develops which may damage the machining electrode. Difficulty in gaining and maintaining the resonant conditions also arises due to the fact that the machining electrode unavoidably is worn during the machining operation. The input frequency to the transducer may be varied in steps but this measure not only renders the arrangement complex and costly but tends to give rise to serious changes in the machining conditions.
From the practical point of view, it should also be noted that needs exist in the industry for achieving a higher goal as far as the EDM efficiency is concerned. Greater machining efficiency is obtainable if machining stability, which is particularly difficult to gain in deep and/or intricate cavity forming, is improved. It has now been observed that with the conventional arrangement, the vibrational activation tends to be restricted on a limited area of the machining surface of the electrode and other areas thereof are left uncontrolled, providing sites for stagnation of the contaminated machining liquid, gases and other machining products and hence contributing to machining instability.